Van Pelt Drive - Transistors Failed

[gregmhoffmann]

So I built the Van Pelt drive and it worked flawlessly for a few days. Last night I left the pedal on and this morning no sound at all. I traced audio through the circuit and it died at Q1, replaced that and found Q2 was bad AND SAME FOR Q3. So after replacing all 3 it works now.

I’m powering it with a Cioks DC-7. All other pedals are fine. Anyone have a clue how that happens? I have about a 5th grade reading level knowledge of circuits so I’m baffled.

 

[Flying]

Hopefully someone with better knowledge than me will chip in, but my guess is that they have been damaged by static electricity. I built a compressor using 2n7000s and that kept breaking because there was no protection, I didn’t know the susceptibility of mosfets to static electricity at the time. It was a magazine DIY project and odd that the designer didn’t build in protection.

However I was under the impression the 9V1 Zener diode in the Zan Pelt was there to protect the first mosfet stage from static electricity and the others were protected by being buffered by the first stage.

I hope someone with real knowledge chips in as it’s damn frustrating when something works then doesn’t!

 

[jwin615]

Fet probably drifted of spec and cause a high voltage spike.
That, or you zapped them all during assembly on a cold day. Then a cascading failure.
I haven't seen fets do this at such low voltage but seen iy plenty at higher voltages.